1. Field of the Invention
This invention is directed to signal detection systems, in general, and to systems which separate and isolate frequency hop signals from interference and other signals, in particular.
2. Prior Art
In a communications system there are several sources of degradation to a signal. One way to overcome the problem is through spreading the energy of the signal across a wide frequency range (referred to as "Spread Spectrum". Frequency hopping is one type of spread spectrum in which the energy of the signal is spread across the frequency range by varying the center frequency of the carrier signal wave. Such a signal is called a "hopper" because the center frequency changes in a more or less random pattern from one location to another and appears to "hop" across the spectrum. Generally, the hop range is in some RF region greater than a gigahertz (GHz) which is 10.sup.9 hertz, such as the X band. Thus, the hop frequency range can be contained in the X band, C band or K band. More generally, the possible hop frequencies for a hopper are DC to any frequency used by man for transmitting information. That part of the spectrum which is useful for transmitting through the atmosphere is usually called RF (Radio Frequency). That is, at lower frequencies it is difficult to transmit without special types of apparatus such as large antennas. Consequently, for most ordinary spread spectrum type communications, the spectrum is the region above 10 Ghz. However, this is limitative because much signal processing (especially in the invention described infra) can be done at IF. That is a local oscillator and filtering devices are used to bring the RF signal down to the IF region where the processing takes place. One of the major problems of frequency hop signals, especially in what is called ESM (i.e. electronic support measures) is monitoring a spectral region and detecting frequency hoppers. It is known that there are several advantages to spreading the spectrum of a signal, i.e. spreading the energy of the signal throughout the spectrum. One advantage is to defeat jamming. A second advantage is to make the signal more convert, in other words to make it more difficult to intercept, to find and to characterize the signal. The prior art devices or systems which detect frequency hop signals, generally, consist of a bank of bandpass filters, one filter at each location where a frequency hopper is expected. By having this bank of filters, the noise and all other errors are filtered out. Typically, this removes wideband interference, but narrowband interference in those particular cells will still show up in the corresponding bandpass filter. In addition, it is expensive to build a bank of bandpass filters.
An approach which might be used (but is not known to exist in the prior art) is to use one bandpass filter, with a variable center frequency which is varied in steps across the entire portion of the spectrum occupied by the frequency hop signal. This is equivalent to scanning with the bandpass filter. The advantage, of course, is that less hardware is required. However, the disadvantage is it takes time to scan across the spectrum whereupon waiting at each frequency might lead to missing something at a different frequency. Also, this technique does not eliminate narrowband interference.
One way to eliminate narrowband interference is to set a threshold. This can be accomplished by some kind of radiometric detection or power measurement which observes the average power across the spectrum and detects anything that is much above average. However, the process would have to define a threshold above which anything would be clipped off. This requires some intelligence, a lot of hardware, complex calculations and the like. This process does eliminate most of the narrowband interference. However, if the system clips off the signal, a little bit of the interference signal is left in the overall spectrum. On the other hand, if the system zeros out the signal (i.e. blanks it out), the system eliminates part of the spectrum which is an undesirable operation. What is desired is to eliminate the narrowband interference without affecting the frequency hopper.